Coordinated expression of 7B2 and alpha MSH in the melanotrope cells of Xenopus laevis. An immunocytochemical and in situ hybridization study.

7B2 is a highly conserved protein present in many secretory cells. Using in situ hybridization techniques and immunocytochemistry, parameters concerning the biosynthesis and storage of the 7B2 protein were studied in the pituitary gland and median eminence of the clawed toad Xenopus laevis, in relation to the physiological process of background adaptation. 7B2-like immunoreactivity was present in the median eminence, in the neural and anterior pituitary lobes and, particularly, in the melanotrope cells of the intermediate pituitary lobe. In these cells, it coexisted with immunoreactivity to proopiomelanocortin (POMC)-derived alpha-melanocyte stimulating hormone (alpha MSH). The melanotropes of black-adapted animals had abundant 7B2-mRNA and POMC-mRNA; melanotropes of white-adapted toads had only low levels of these mRNAs. The presence of 7B2 in nerve terminals and endocrine cells supports the idea that the protein has a general function in the cellular secretory process. In X. laevis, 7B2 appears to be particularly associated with POMC and alpha MSH and, therefore, may play a role in the regulation of background adaptation.